This invention relates generally to transformers, and more particularly to radio frequency coupled-transmission-line impedance transformers.
There are many applications wherein it is desired to transform electrical impedances from one value to another. One such application is the matching of the input and output impedances of a microwave power device to a signal generating source and a load respectively.
Several techniques for providing impedance transformation at radio frequencies are known. The conventional approach used in microstrip and stripline matching of the low impedances of microwave power devices to respective input and output circuits involve the use of step transformers, such as described in "Microwave Filters, Impedance-Matching Networks and Coupling Structures" by Matthaei, Young and Jones, McGraw Hill Book Co., 1964. Other techniques include systems utilizing tuned transformers and toroidal ferrite transformers.
The technique of using stepped transformers often need transmission line sections with characteristic impedances of as low as one or two ohms. Thus, in microstrip or strip-line construction, such low characteristic impedances can only be realized with unreasonably wide lines that create discontinuities which are difficult to characterize. Also, the lack of convenient tuning schemes in such transformers often makes minor circuit adjustments difficult.
Whereas the latter techniques provide a way to match the input and output impedances of a microwave power device to its associated input and output circuitry, the use of tuned transformers is not suitable for wide band applications. The toroidal ferrite transformer used is expensive to manufacture and becomes inefficient at higher frequencies.
The need exists for coupled-transmission-line impedance transformers for high-ratio impedance transformations in microstrip or stripline without the ordinary difficulties encountered in conventional impedance-transformation schemes.